Recently, in the motor vehicle industry, needs for displaying a monitor video camera image for a driver on a front-seat monitor and also arbitrarily displaying a DVD image and a car navigation image (hereinafter called “navigation image” on both front-seat and back-seat monitors have increased.
In order to arbitrarily display a plurality of video channels on respective front and back monitors, it is necessary to display both a natural image, such as a DVD image and the like and a CG image/line image, such as a navigation image and the like with high definition, using a vehicle-mounted LAN.
Simultaneously, since a LAN mounted on a current vehicle has a video signal transmission capacity for at most only one channel, it is necessary to compress the image data of both a natural image and a CG image with high efficiency/low transmission delay without the degradation of definition.
Furthermore, for a vehicle-mounted system, it is necessary that the circuit scale of a video system is small.
Although there are various standardization methods in video compression, it is difficult to obtain high definition in both a natural image, such as a DVD image and the like and a CG image/line image, such as a navigation image and the like. A video compression method, such as JPEG, MPEG or the like can efficiently display an image having a smooth gradation change. However, in order to clearly display an image having a steep gradation change like an edge, a large amount of codes is necessary, thereby deteriorating video compression efficiency.
Of navigation images, an image having a texture pattern, such as checks, irregular salt and sesame noise in the background is most problematic with respect to definition. Generally, although texture is periodic, it has an irregular shape and is difficult to compress. Therefore, there is a tendency that the amount of codes widely increases. In the case of a transmission path requiring band assurance like a vehicle-mounted LAN, it is necessary to suppress to a specific amount of codes by compressing image data. However, when a large amount of codes are used to display texture, it becomes necessary to increase the data compression ratio in that part. Namely, data compression is increased at the sacrifice of definition. Therefore, it is a problem how an image can be efficiently encoded in a texture area. However, lossy coding in which such loss of image information that cannot be easily sensed by human eyes is allowed is effective in the display of a natural image, such as a DVD image. More particularly, a prediction encoding method for predicting a current pixel using the known values of adjacent pixels, quantizing a prediction error value between the predicted value and the current pixel to make variable-length coding is known as very effective image compression method for displaying a natural image since its circuit scale is small.
However, the prediction coding method predicts a current pixel from several adjacent pixels before and after the current pixel. Therefore, an image having a cycle of several tens of pixels and a steep edge (image area having a steep gradation change) is difficult to predict. As a result, the prediction error value becomes large and a large amount of codes is required to encode such a large prediction error value, which is a problem.
As a method for compressing a periodic image pattern, such as a texture pattern, conventionally a technique for compressing a dot photographic image and a design image is known. A prior art for compressing a periodic image pattern will be explained below.
Non-patent document 1 discloses an application technique of a prediction encoding method. The technique is provided with a plurality of predictors matched with the dot cycle of a reference pixel in a compression method of a dot picture or the standardization method JBIG of a black/white binary image compression for a newspaper image and the like and selects a prediction result having the least prediction error for each specific section. However, since this method has a plurality of predictors, its circuit scale becomes large, which is a problem. Although a dot pattern is a kind of a texture pattern, its size varies. Therefore, the pattern has a character different from the texture pattern in a navigation image and the like. Therefore, this prior art is not always effective in the compression of a texture pattern in a navigation image and the like.
Patent document 1 discloses a universal dictionary type compression method (model LZ77) for detecting a repetitive series that has appeared before and compressing an image by transmitting the matched position and length of the repetition. This method is lossless coding. Its hardware is realized using CAM (content associative memory) and can encodes not only a periodic pattern but also a general image pattern. However, this method has a problem that both its circuit scale and process load are large.
Patent documents 2 and 3 disclose a method for detecting whether the same series as the current one has appeared before and encoding the number M of one cycle and the times of its repetition and a method for detecting whether the same series as the current one has appeared before and encoding its matched off-set position and length, respectively. These methods are lossless coding and are realized by simplifying the LZ77 dictionary type compression method. However, the methods have a problem that they are not suitable for the compression of non-periodic image pattern, such as a natural image.
Any of the prior arts disclosed by Patent documents 1 through 3 is lossless coding, in which as illustrated in FIG. 1, a matching detection circuit 102 detects the matching between a repetitive series that has appeared before and a repetitive series that currently appear while image data is accumulated in the memory 101 for the previous series. Then, a variable-length encoding circuit 103 encodes information about the matching by variable length and outputs compressed codes. As understood from this configuration, any of the above-described prior arts does not adopt a configuration for performing prediction encoding suitable for the compression of a natural image and high-efficiency coding cannot be performed in an environment where a natural image and a navigation image are mixed and a vehicle-mounted LAN cannot be anticipated.
Patent document 4 discloses a compression technique for an image to which a design is applied. However, this method also has a problem that it is not suitable for the compression of a non-periodic image pattern, such as a natural image.
The following documents are publicly known examples that are referenced in the specification of the present invention.    Non-patent document 1: “Data Compression of Newspaper Pages Using Adaptive Prediction” (Technical Report of The Institute of Electronics, Information and Communication Engineers IE78-55)    Patent document 1: Japanese Patent No. 2713369    Patent document 2: Japanese Laid-open Patent Publication No. 2003-264703    Patent document 3: Japanese Laid-open Patent Publication No. 11-168389    Patent document 4: Japanese Laid-open Patent Publication No. 2003-174564